6.19 Arrays of Variable Length

Variable-length automatic arrays are allowed in ISO C99, and as an
extension GCC accepts them in C90 mode and in C++. These arrays are
declared like any other automatic arrays, but with a length that is not
a constant expression. The storage is allocated at the point of
declaration and deallocated when the brace-level is exited. For
example:

Jumping or breaking out of the scope of the array name deallocates the
storage. Jumping into the scope is not allowed; you get an error
message for it.

You can use the function alloca to get an effect much like
variable-length arrays. The function alloca is available in
many other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.

There are other differences between these two methods. Space allocated
with alloca exists until the containing function returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends. (If you use both variable-length arrays and
alloca in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with alloca.)

You can also use variable-length arrays as arguments to functions:

struct entry
tester (int len, char data[len][len])
{
/* ... */
}

The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
sizeof.

If you want to pass the array first and the length afterward, you can
use a forward declaration in the parameter list—another GNU extension.

The `int len' before the semicolon is a parameter forward
declaration, and it serves the purpose of making the name len
known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the
parameter list. They can be separated by commas or semicolons, but the
last one must end with a semicolon, which is followed by the “real”
parameter declarations. Each forward declaration must match a “real”
declaration in parameter name and data type. ISO C99 does not support
parameter forward declarations.